Rotary evaporator. Chemist taking the temperature of the water bath used with a rotary evaporator. This device allows crystallisation of a compound from a solution at low temperature. The solution is in the round bottom flask in the hot water bath at lower right. A vacuum pump (upper centre) lowers the pressure in the flask, allowing it to boil. The flask is rotated to prevent violent boiling. The removed solvent is collected in the flask at lower left, leaving the crystallised product in the flask in the water bath. This technique is used when the product would be damaged by the high temperatures associated with boiling the solvent off at atmospheric pressure.